arm: mvebu: clearfog: update eMMC documentation

SPL now automatically selects the correct U-Boot image offset for both
eMMC and SD card. No need to tweak
CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR anymore.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>